GI5
Studies on Silver Goulometer
by
0. D. Buckner.
N?
UNIVERSITY OF CALIFORNIA
AT LOS ANGELES
8061 'IZ NVf 1W
japu.ig
Studies on
the Silver Coulometer
A DISSERTATION
PRESENTED TO THE
FACULTY OF PRINCETON UNIVERSITY
IN CANDIDACY FOR THE DEGREE
OF DOCTOR OF PHILOSOPHY
G. D. BUCKNER
ACCEPTED BY THE DEPARTMENT OF CHEMISTRY
JUNE, 1912.
paper presented at the XXII General
Meeting of the American Electrochem-
ical Society, in Joint Session with
Sections II: Inorganic Chemistry, Xa:
Electrochemistry, and Xb: Physical
Chemistry, of the VIII International
Congress of Applied Chemistry, in New
York City, September 9, 1912. Dr. W.
Lash Miller in the Chair.
STUDIES ON THE SILVER COULOMETER*
By G. D. BUCKNER.
Introduction.
Previous to the starting of this investigation there had been
extensive studies and experiments made with the silver cou-
lometer, dating back to the fundamental work of Rayleigh and
Sedgwick 1 in 1883, when the coulometer first became an accurate
instrument for the measure of electric current. The present
investigation was undertaken in order to throw light upon a
matter which had received no attention in the numerous articles
published on the silver coulometer.
In the discussion which followed Duschak and Hulett's 2 work
on the silver coulometer the point was made by Mr. Hering 3
that, since platinum is known to contain oxygen in some form,
it seems possible that dissolved oxygen may play some part in
the deposition of silver, and it would naturally follow that
abnormal weights might be caused thereby.
Platinum is known to take up oxygen either in solution or as a
compound, forming an oxygen electrode in equilibrium with the
oxygen in the electrolyte. On passing a current through a
solution of silver nitrate the main reaction at the cathode would
be the deposition of silver, but some of the current would go to
the removing of the oxygen, and, since 8 grams of oxygen are
equivalent to 107.88 grams of silver, it follows that there might
be a deficit in the amount of silver deposited, an amount which
might easily admit of measurement. It seemed possible that
* The work described in this article was made possible by a grant from the
Elizabeth Thompson Science Fund, which placed at our disposal some of the special
apparatus and materials needed for this investigation, and the authors take pleasure
in acknowledging their indebtedness to the Trustees of the Fund.
1 The literature of the coulometer is particularly reviewed by Guthe, Bull. No. 3,
Bureau of Standards, 1905, page 347. and by Smith, Mather and Lowrv. Phil.
Trans., 207, 545. Rayleigh and Sedgwick; Phil. Trans., 175, in (1884). '
2 Duschak and Hulett: Trans. Am. Electrochem. Soc., 12, 257 (1907).
3 Hering: Trans. Am. Electrochem. Soc., 12, 293 (1907); Bose: Chem. Ztg., 26,
67 (1902).
18946C
2 G. D. BUCKNER.
information on this question could be obtained by using as a
cathode cup some metal such as gold, which does not measurably
absorb oxygen.
Considerable work has been done on the silver coulometer in
vacuo and in solutions saturated with nitrogen, which means
the exclusion of oxygen, and, while the evidence is not con-
clusive (most experimenters have observed a heavier deposit
where oxygen has been excluded), this might be explained by
the foregoing assumption.
The type of coulometer used by us was essentially the same
as that used by Duschak and Hulett. Two of the cups were
made of gold and two of platinum, each containing a porous
cup to retain the heavy anode liquid. These were run in series,
with the idea of comparing the silver deposited on platinum and
on gold, in order to determine whether gold could be substituted
for platinum as a cathode and thereby overcome the influence
which might be caused by the oxygen in the, platinum.
We soon found that the purity of the materials used in making
up the electrolyte affected the deposits, and also that the inclu-
sions in the deposited silver had to be taken into consideration,
so this investigation really involved several important questions.
and has resulted in two independent researches. Since it soon
developed that the slight impurities in the electrolyte caused
measurable variations in the weight of the silver deposited, it
seemed probable that this variation in the weights might be due
to the variation in the impurity included by the deposited silver.
A very careful study of inclusions in electrolytic silver was taken
up by Mr. J. S. Laird, and a method developed for the accurate
determination of these inclusions, as described in a separate paper.
Our first problem was to get a standard reproducible silver nitrate
solution.
Incidentally we obtained some information from the suggestions
of Richards 4 and others that a complex ion is formed at the
anode, and that when this anolyte comes in contact with the
cathode the deposit of silver is too heavy. In testing this point
we arranged two coulometers in series, in which we maintained
by the use of syphons the catholyte and anolyte at such levels
that in each cathode cup the flow of the liquid would be towards
the opposite pole.
4 Richards. Collins and Heimrod; Proc. Am. Acad., 35, 123 (1899).
THE SILVER COULOMETER. 3
We also attempted to substitute "Alundum" cups made of
pure A1 2 O 3 for the porous porcelain cups, since these permitted
an easier flow of liquid and lessened the resistance.
In series with most of these experiments we ran a coulometer
of the Rayleigh type, using the filter paper inclosed anode. We
also experimented with electrolytes of different degrees of purity.
Apparatus Used.
Each cathode cup was 10 cm. high, 5 cm. in diameter, and
weighed 72 grams. The cups were so adjusted as to weigh within
o.ooi gram of each other, the advantage of this being that one
tare would suffice for all of the cups and the exact differences
in weight could be determined with the rider. The cathode cups
Nos. i and 2 were of platinum, while 5 and 4 were of pure gold.
These cathode cups were handled with tongs at all times. They
were thoroughly cleaned with sea sand, also chemically, and
washed, after which they were heated to redness for ten minutes
in an electric furnace. This final treatment produced a surface
on the gold cups to which the silver adhered so tenaciously that
it could not be readily detached. Since the deposits were to be
saved, this treatment was abandoned. When, however, the
cathode cups were placed in wooden molds, and the internal
surfaces made smooth with a blood-stone burnisher, then cleaned
as stated above and heated in an electric furnace at 160 for
thirty minutes, it was found that this: treatment gave a gold sur-
face from which the silver deposits could generally be removed
with a platinum spatula. The deposits in the platinum cups could
always be removed with ease.
The weighings were made under the same conditions as those
described by Duschak and Hulett, the only difference being
that the swing divisions were read through a small telescope
fixed in the balance case. Duplicate weighings could be made
to 0.02 mg. with certainty. Care was always taken that tem-
perature equilibrium had been established in the balance case,
and the cups were always placed on the pans with long tongs.
The weights were calibrated, and at each weighing the tempera-
ture, barometer and hygrometer were noted. The anodes were
made of "atomic-weight" silver, the purification of which is
described elsewhere. The silver was melted in a porcelain cru-
G. D. BUCKNER.
cible and cast into cylindrical sticks 6 cm. by i cm. in a mold
made of pure graphite which had previously been heated to
redness for thirty minutes. These anodes were suspended by
platinum wires which served as electrical connections. The four
cathode cups were cleaned, adjusted to the same weight by re-
moving a little from the top of the heavier ones, cleaned as
described above, cooled in separate vacuum desiccators, and
finally weighed.
=:-_=-=
___
FIG. i.
The cathode cups were now set up ready for a run, on circular
pieces of plate glass the bottoms of which had been previously
paraffined and on the top of which had been placed several
pieces of filter paper to insure perfect insulation. The cathode
cups (a) as shown in Fig. i were surrounded by glass jackets (b )
which extended 2.5 cm. above the cups, and the glass cylinders
were covered with pieces of glass provided with two holes, those
in the center being just large enough to hold the porous cups (c),
THE SILVER COULOMETER. 5
while those at the side and through which the catholyte was intro-
duced were smaller. On the glass cover rested a hard rubber
bridge (d) from which the anode (e) was suspended as indicated
in the figure. In series with these four coulometers we ran one
of the Rayleigh type, modeled after the specifications prepared
by the National Academy of France. The Rayleigh bowl was
9 cm. by 4 cm. deep.
The porous cups were exact copies in size and texture of the
Berlin "Pukel" filter tubes except that the upper 3 cm. were
vitreous. They were 2.5 cm. in diameter by 12 cm. long, and
when placed in position extended to within 0.75 cm. of the bottom
of the cathode cups. These porous cups were treated with con-
centrated nitric acid, and then water was allowed to flow through
them continuously until the last trace of nitric acid had been
removed. At no time were they glowed or allowed to dry out.
During a run the anolyte was removed at intervals with a constant
level pipette and the catholyte added, so that the electrolyte was
always flowing into the porous cups towards the anode. After
an experiment had been completed the silver crystals were
loosened with a platinum spatula and sealed in test tubes which
had previously been glowed. This silver was examined by Mr.
Laird, as described in another communication.
Materials Used.
In previous work \ve learned that filter paper was to be avoided,
and in the present investigation we soon found that not only filter
paper, but dust, organic matter of every kind and even dissolved
glass so affected a silver nitrate solution that it gave abnormally
heavy deposits. This research is therefore largely devoted to the
question of the purity of the materials used in coulometer work.
In order to have a reliable and reproducible basis of com-
parison it was found necessary to work with a silver nitrate
solution made in a definite way, which was essentially the same
as that used by Richards in preparing the silver nitrate from
which he determined the atomic weight of silver. This must
give us a solution in which the equivalence between Ag and NO 3
is exactly equal, provided we rigorously exclude organic matter
in all subsequent operations. We therefore paid particular atten-
tion to the elimination of all organic matter from our water, and
6 ('.. I). HL'CKX KK.
used only quartz, gold and platinum vessels, so that there could
be no possible reduction of the silver nitrate by dissolved glass
or by any other reducing agent. This is our "normal" coulometer
electrolyte.
Preparation of Silver.
The first step in purifying silver is to obtain it in the form of
silver chloride. Richards 5 digested silver chloride in a solution
of potassium hydroxide and sugar, in order to obtain metallic
silver. If, however, the silver chloride is dissolved in ammonium
hydroxide and filtered, certain substances are eliminated, and
there is an advantage in reducing silver directly from a solution
rather than from the solid. We encountered a difficulty in reduc-
ing the silver from an ammoniacal solution with hydroxide and
sugar in a desirable crystalline form, but found that this could
be done as follows : Silver was obtained from the U. S. Mint,
dissolved in nitric acid and precipitated as silver chloride. It
was then digested in aqua regia for several hours on the water
bath, filtered and washed. The silver chloride was dissolved
nearly to saturation in a closed container in ammonium hydroxide
of specific gravity 0.93. Five hundred c.c. of this silver-
ammonium chloride solution was then decanted and filtered into
a liter flask which contained 25 g. of cane sugar dissolved in
50 c.c. of water. Silver did not appear at first, but when the s
solution was heated to incipient boiling the silver began to separate
in a very finely crystalline form. The ammonia which distilled
was led into pure water for future use. As the reduction pro-
ceeded a solution containing 100 g. of sugar and 25 g. of potas-
sium hydrate in 150 c.c. of water was added through a dropping
funnel, rapidly at first and more slowly as the evolution of
ammonia becomes less. After the sugar solution had been added
and the evolution of ammonia had ceased, the liquid was vigor-
ously boiled for fifteen minutes. Under these conditions we
were able to reduce silver from an ammoniacal solution in a
crystalline form which could be readily washed. It was found
to be finely granular, and contained only exceptionally the
slightest 'traces of silver .chloride, which were entirely removed
by shaking thoroughly with ammonium hydroxide. The small-
grained silver possessed the virtue of lessening the possibility of
"Richards and Wells; Jour. Chem. Soc., 27, 475 (1905).
THE SILVER COULOMETER. 7
silver chloride and other impurities being occluded, it was easily
soluble in nitric acid, and the solution gave no cloudiness on
dilution. The silver was now placed in a Jena flask, dissolved
in nitric acid, the solution evaporated, and the residue fused in an
electric furnace. The fused silver nitrate was dissolved in "con-
ductivity" water, and the metal precipitated with ammonium
formate according to the directions of Richards and Wells. This
silver was composed of large crystals, and was as pure as "atomic-
weight" silver. It was melted and cast into molds, as has been
already described. This silver was used in the preparation of
our "normal" electrolyte.
Purification of Water.
Since dust, organic matter or dissolved glass in the water would
partially reduce our purest silver nitrate, particular care was
exercised in purifying the water employed in our work. The
apparatus consisted of a 1 5-liter Jena retort, the neck of which
was drawn down and cut off so as to just fit a quartz condenser
tube ground into and extending into the retort. The arrange-
ment was such that only vapor could enter the condenser, the
water separating on the glass simply flowing back into the boiling
liquid. Air filtered through absorbent cotton and deprived of all
organic matter by passage over a glowing platinum spiral, wound
evenly on a porcelain tube, was bubbled slowly through the gently
boiling water. This water contained a small amount of KMnO 4
and Ba(OH) 2 and was freshly prepared conductivity water.
The whole apparatus was drained for fifteen minutes before any
water was condensed, only the second third being caught and
retained in a quartz flask. Since this water had been in contact
with quartz only, it must have been free from all traces of dust
and organic matter.
Preparation of Silver Nitrate and Nitric Acid.
The nitric acid was purified according to a method identical
in the main to the one employed in obtaining pure water. No
air, however, was bubbled through the acid during its distillation.
The purest silver was dissolved in this nitric acid and water
prepared by the foregoing method. The quartz flask was placed
in an electric furnace where the temperature was accurately' con-
8 G. D. BUCKNER.
trolled, the solution was evaporated, and the residue fused ac-
cording to Richards' directions until the last trace of nitric acid
had been expelled. Extending down into the quartz flask which
contained the silver nitrate was a porcelain tube through which
passed air purified as previously described. The silver nitrate
was kept covered, in a dark place, until used the day following,
and the water was prepared and the solution made just before the
experiment was started. A 15 percent solution was used in all
experiments. We regarded this as the purest obtainable silver
nitrate solution on a reproducible basis.
Manipulation.
After obtaining the accurate weights of the cathode cups they
were placed on the circular glasses and filter papers with a strip
of platinum foil under each cup, which served as a conductor
for the electric current. The glass jackets were placed over
them, and the silver nitrate solution, prepared as above, was
added. The porous cups were now rinsed with silver nitrate
solution and placed in position; thus the catholyte began im-
mediately to diffuse into them. The anodes were fixed in place,
and then the anolyte was added, but only to such a level that
the catholyte was always passing into the anode chamber. After
a current of about 0.3 ampere had passed through the coulometers
for about four hours, depositing approximately 5 grams of silver
in each cup, the current was broken and the anodes were removed.
The anolyte was drawn off with a pipette, and following this
the catholyte was removed by the same method. Finally the
porous cups were removed and examined for small particles
of loose silver which might have adhered to them. The anolyte
and catholyte were reserved in separate flasks, and, although
all solutions were tested for acidity, they were always found to
be neutral.
The cathode cups were allowed to drain into small crystallizing
dishes, each inclined against a glass support. The cups were
next filled with distilled water, which was removed with a freshly-
blown bulb pipette, run into a marked flask, and the cathode
cups were drained again into the same crystallizing dishes. This
was repeated three times, until the water which had stood over
night in the cups gave no test for silver with KI. It was quite
THE SILVER COULOMETER.
impossible for any loose silver to escape us in this way, for
previously it was shown that as little as 0.002 mg. 2 of silver
could be detected in a flask and recovered. The cathode cups
were always handled with tongs, wound with linen thread, so
that no dust or liquids came in contact with the outside of the
cups, which were always bright and polished. The loose silver
was placed in the proper cathode cups, and these were dried in
vacuum desiccators and weighed. We also satisfied ourselves that
there was no appreciable change in weight when the cathode cups
with the deposits were subsequently heated to 160 for thirty
minutes. In the experiments made using the two platinum and
two gold cathode cups a "Rayleigh" form was also run in the
series. The results are as follows (Table I) :
TABLE I.
Run
On
Pt.
Differ-
On
AU
Differ-
No.
i
2
ing.
3
4
mg.
7
8
9
10
ii
5.83369
547605
5-59653
4.98000
6.26536
5.83383
547586
5-59663
4-97953
6.26540
0.14
O.I9
0.10
0-47
0.04
5-83449
lost
5-59756
4.98036
6.26589
5-83444
5-47637
5-59746
4.98038
6.26603
05
.10
.02
.14
Deposit on
Rayleigh
Run Average
Average
'Rayleigh' Gold heavier | deposit
No. Pt. deposit
Au deposit
deposit than on
heavier than
Pt. mg.
on Pt. mg.
7
5.83376
5-83446
5.83525
+0.70
+1-49
8
5-47595
547637
5-47749
+0.42
-f- 1 -54
9
5.59658
5-59751
5.59848
+0.93
4-1.90
IO
4.97976
4.98037
4.98184
-fo.6i
+2.08
II
6.26538
6.26596
6.26661
+0.58
+1.23
If we calculate the percentage difference in the weight of
silver deposited on platinum and on gold by the same current,
we find that the deposits on the gold are 0.012 0.0012 percent
heavier than the deposits on platinum, while in the Rayleigh
coulometer, where Kahlbaum's C. P. silver nitrate and the filter
paper enclosed anode were used, we found the deposits to be 0.030
. 0.0033 percent heavier than the deposits made on the normal
platinum cathode cup. In every case it was found that the silver
10 G. D. BUCKNER.
deposited on gold was heavier than that deposited on platinum
by the same current, the average excess being 12 parts in 100,000.
This could be explained by assuming that part of the current in
the platinum coulometers passed from the catholyte to the plat-
inum cathode by removing oxygen from the platinum.
The silver samples reserved from these runs were investigated
by Mr. Laird, but before the method of determining the conclu-
sions was in its final state, the deposits from the gold cups
showed in all cases slightly more inclusions and those from the
Rayleigh form very much more than did the deposits from the
platinum cups. The difference in the amounts of inclusions in
the silver from the platinum and gold may be due to the fact
that the silver crystals deposited on the gold were close together
and nearly covered with gold, whereas those on the platinum
were isolated.
These deposits on the platinum also contained impurities, so
in our judgment it matters little which metal we use as cathode
when the exact electrochemical equivalent of silver is to be deter-
mined, for the impurities in the silver deposit must be determined
in any case if an accuracy greater than i in 5,000 is desired.
In examining the deposits we found that the silver crystals
deposited on the platinum cathode were rather large and well
defined, being placed irregularly on the sides of the cathode cups,
with only a few on the bottom. Only in very few cases was
there a tendency towards a striated arrangement of the crystals
on the platinum, and in all cases the silver was easily detached
by a platinum spatula without appreciably changing the weight
of the cups. One of the difficulties encountered in using a
platinum cathode was the fact that there was always a small
amount of loose silver present which had to be recovered.
It seemed to us that gold might be a more desirable metal
for the cathode cups than platinum, and when the surface of the
gold cups was slightly rough there was generally no loose silver,
but sometimes there was, for the adhesion of the silver crystals
to the gold cups was not sufficient invariably to retain all of
the silver during the manipulations. However, the adhesion was
sufficient to make it a very difficult matter to remove the silver
for examination as to inclusions. Gold is a soft metal, and the
thin-walled cups were difficult to handle. Since the silver crystals
THE SILVER COULOMETER. 1 1
completely covered certain parts of the gold cathodes, and there
may have been more chance for inclusions between the silver
and gold than between the silver and platinum in the platinum
cathodes, the platinum cups appear on the whole to be superior
to those of gold, and if the dissolved oxygen plays any role
it is only a few parts in 100,000.
The deposits in the "Rayleigh" form were feathery and
striated at all times. The comparative character of the deposits
in the platinum and gold cups can be seen in Fig. 2.
FIG. 2. The gold cup on the right shows the small Ag crystals as compared with
those deposited on a platinum cup as shown on the left.
The Effect of Solutions of Knoivn Purity.
Since the impurities in the deposits caused the abnormal
weights, we decided to try some experiments in which the silver
nitrate solutions in the cathode cups were of different but of
known sources. In platinum cup No. I we used our "normal"
solutions, in No. 2 a solution of Kahlbaum's C. P. silver nitrate
which had been recrystallized after adding I c.c. of our purest
nitric acid to the solution, decanting, evaporating and fusing the
salt as directed above. The manipulation was carried out in
detail as in the previous experiment. In the last four experiments
we also ran a "Rayleigh" type which contained the same re-
crystallized silver nitrate solution contained in cup No. 2. The
results were as follows :
12
G. D. BUCKNER.
TABUS II.
Run
No.
"Normal"
A
ing.
Kahlbaum's
mg. Heavier
than Normal
Rayleigh
Kahlbaum's
12
13
14
15
16
17
18
19
20
7-I2I75
4-96636
5.97746
6.66X93
572269
6.04IOI
5-040/3
5.77169
0.58
0.76
0.82
0.29
1.61
0.41
0.30
1.09
6^66222
5-72430
6.04142
5.04103
5-77060
2.99
3-19
1-53
3-18
2.09
6.66492
572588
6.04254
5.0439I
5-77378
In experiment 20 the silver deposit in coulometer No. 2 was
partially discolored, and, while the cause is unknown, it was
obviously to be rejected, although we proceeded with the weigh-
ing as a matter of form. Referring to table No. 2, coulometer
No. i always contained our "normal" silver nitrate solution, while
in No. 2, which was like No. I in all other respects, was placed
the solution made from our best water and Kahlbaum's silver
nitrate, which had been treated with a little nitric acid, re-
crystallized and fused as previously described. This same solu-
tion was also used in the Rayleigh form. We expected to find
that the Kahlbaum's C. P. silver nitrate when treated in this way
would give the same results as our "normal" solution, but we
found that coulometer No. 2 in every case gave a heavier deposit,
the average excess being 0.009 percent. We noticed also that
the Rayleigh form with the same solution as that in coulometer
No. 2 gave a deposit which was always much heavier than that
obtained from our normal electrolyte, the average excess being
0.036 percent. Furthermore, the deposits from coulometer No. 2
show distinctly more inclusions than those from coulometer No. I,
while the deposits from the Rayleigh show still greater amounts
of inclusions than from No. 2.
It would follow, therefore, that the source and previous history
of the silver nitrate solution always plays a measurable role
in the weight of silver deposited by a given number of coulombs.
We are therefore forced to the conclusion that it is necessary to
determine and allow for the inclusions in the deposited silver
when any attempt at great accuracy is made, and, although
THE SILVER COULOMETER. 13
reasonably pure materials for the electrolytes will suffice, the
purer the solutions used the less the error due to inclusions,
which is as small as one part in 10,000 for our "normal" electro-
lyte in a porous cup platinum coulometer, where the anolyte
has no access to 'the catholyte during the electrolysis.
Effect Due to the Anolyte.
It has been maintained by Richards and others that the solu-
tion formed at the silver anode in a silver nitrate solution is
abnormal in, that it will yield an abnormally heavy deposit when
used as catholyte. Table II clearly shows that the Rayleigh
form gives a markedly heavier deposit than does the porous cup
coulometer with the same solution. Filter paper has the property
of so changing a silver nitrate solution that it gives a heavier
deposit, a fact which in part or wholly would account for the
difference between the results obtained with coulometer No. 2
and the Rayleigh form, as shown in Table II. There is another
possible factor. In the Rayleigh coulometer the silver nitrate
formed at the anode increases the density of the solution at the
bowl, and it flows down through the filter paper and comes in
contact with the cathode. In order to get some evidence on the
possible effect of the anolyte we carried out the following
experiments :
In our porous cup coulometers /, 2, 5 and 4, filter paper and
all organic matter were excluded; furthermore, the anolytes in
the porous cups were always maintained at a lower level than
the catholytes, so that the flow of liquid was from the catholyte
to the anolyte. Now, by keeping the electrolyte at a higher
level in the anode than in the cathode cups, we can cause the
anolyte to flow through the porous cups into the catholyte.
We ran two coulometers in series. From No. I we removed
the anolyte by a constant level syphon which extended to the
bottom of the porcelain cup, thereby removing the heaviest liquid
as it collected at the bottom. In No. 2 the catholyte was removed
by a quartz syphon which only extended below the top of the
catholyte, thereby removing only the lighter and allowing the
heavy liquid to flow through the porous cup and collect at the
bottom of the cathode cup. The electrolytes were our "normal"
silver nitrate solutions, and all details of the two coulometers
G. D. BUCKNER.
were identical except the direction of the flow of the electrolyte.
The results were as follows :
TABLE III.
Run
Catholyte
Anolyte
Difference
No.
removed
removed
mg.
32
4.66l8l
4.66079
+1.02
33
4-I3056
4.12998
+0.58
34
3-75443
3-75361
+0.82
35
5-12350
5.12286
+0.64
36
4.64026
4.63926
+ 1.00
These results show that the heavy anolyte does play some role
in the formation of the heavier deposits obtained. These five
results indicate an increase in weight of the silver deposited of
0.019 percent, due to the presence of the anolyte during elec-
trolysis. This is approximately half of the increase in weight
(0.036 percent) observed in the Rayleigh form, where the anolyte
not only flows into the catholyte, but filter paper also is present.
Porous Cups of Different Material.
The porous cups with vitreous upper portion, which we used,
were entirely satisfactory. They offered, however, more resist-
ance to the current than was desired, and it took a considerable
length of time to wash them absolutely free of acids. For other
reasons it seemed desirable also to test porous cups made of some
other material than porcelain, and we selected "alundum," which
can be made into excellent filtering tubes. It did not seem con-
ceivable that crystalline alumina could affect a silver nitrate solu-
tion. These tubes were of the same dimensions as the porous
cups, only thin walled (1.5 mm.), allowed the liquid to filter
through much more readily, and could be washed thoroughly in a
short time. Four alundum cups having the above dimensions
were obtained and thoroughly cleaned with nitric acid and water.
Experiments 21, 22, 23 and 24 of Table IV were made with these
substitutes for the porous porcelain cups, the normal electrolyte
being used. The catholyte and anolyte were maintained at the
same level. Here the agreement of the results obtained with the
platinum cups No. / and 2 is particularly good, and a comparison
THE SILVER COULOMETER.
of the results obtained with the gold cups 3 and 4 is also good;
but here again we find the heavier deposits in the gold cups. In
run 23 there was a very fine scum floating on the catholyte in
the platinum cups which could not be accounted for. The run,
however, was completed as in all cases. These alundum cups
retained the anode slime perfectly, but are quite porous, and no
doubt some of the anolyte diffused through them to the catholyte.
We therefore made six experiments with the two platinum cups.
Coulometer No. / was provided with an alundum porous cup
in which the anolyte was at the same level as that in the cathode
chamber, while No. 2 contained the original porcelain cup where
the anolyte was at a lower level and there was no diffusion in to
the catholyte. In every case except in 27 the coulometer with
the alundum cup showed a heavier deposit. The average is 0.026
percent. The results are as follows :
TABU; IV.
Run
No.
i
Alundum
Differ-
ence
mg.
2
Alundum
Gold heav-
ier than
Plat. mg.
Alundum
Differ-
ence
mg.
4
Alundum
21
22
23
24
2<5
5.88568
5-33804
5-7II43
4.42802
4.71515
0.31
05
05
-03
-j-o 03
5-88537
5-33809
57II38
4-42799
Porcel'n
4 71518
+2.06
+0.59
0.49
+0.26
5.88766
5 33874
5.71104
4.42820
0.16
0.16
0.22
O.06
5-88750
5.33858
571082
4.42826
ofi
4 92720
-\-o 19
4 92706
27
4.36494
o 14
4 36508
"8
4 5 2 397
-J-o 28
29
5.11922
-j-o 31
5 i 1891
30
3.83131
+O.II
3 83120
The effect here is, no doubt, due to the anolyte diffusing
through the porous alundum cup, and, while we could arrange
syphons to keep it slightly lower and cause the flow to be into
the anolyte, the< use of syphons makes the apparatus complicated
and causes the failure of a larger percent of the experiments
than occurs when the denser, less porous porcelain cups are
employed, where it sufficed to remove a little of the anolyte with
a small bulb pipette from time to time, which is a simple opera-
tion, and, besides, there was no diffusion through the cup.
Alundum cups could be made of the same texture as the porcelain
16 G. D. BUCKNEJR.
cups, but then they would lose the advantage of being easily
washed. Our porcelain cups with vitreous upper portions were
distinctly superior to the "alundum" cups, and the coulometers
as used were made for simplicity and exactness in manipulation,
and were quite free from sources of accidental error.
General Summary.
1. It was found that the slightest traces of impurities, such as
dust and organic matter, so affected a silver nitrate solution that
it gave abnormally heavy deposits. Particular attention was paid
to preparing a reproducible "normal electrolyte." It was necessary
to take unusual precaution in preparing all materials, and we used
only quartz, gold and platinum utensils.
2. In preparing "atomic-weight" silver the usual method was
modified. We found the conditions under which silver could
be precipitated from an ammoniacal solution of silver chloride in
a crystalline condition, which is a distinct advantage over reducing
solid silver chloride with sugar and caustic potash.
3. In preparing pure water, particular attention was paid to
the exclusion of dust and organic matter of all kinds, even to the
burning of volatile organic matter which might be present in
the air that came in contact with the solutions used. Since the
solubility of glass suffices to affect measurably a silver nitrate
solution, the water employed in our work was only permitted to
come in contact with quartz.
4. Silver nitrate was obtained from "atomic-weight" silver
and pure nitric acid, the excess of acid being driven off at a
definite temperature in an electric furnace, and with the exclusion
of all organic matter. With this silver nitrate a solution was
prepared which contained exact equivalents of Ag and NO 3 .
Kahlbaum's C. P. silver nitrate, recrystallized and fused, gave a
deposit 0.009 percent heavier than that obtained with our normal
electrolyte, and other solutions yielded distinctly heavier deposits.
5. Four porous-cup coulometers were run in series. The
cathode cups in I and 2 were of platinum, while 3 and 4 were
gold. They contained the same solutions and were treated in
exactly the same manner. In every case we found the deposits
on gold to be heavier than those on platinum, the average excess
being 12 parts in 100,000.
THE SILVER COULOMETER. 17
6. In experiments with a "Rayleigh" form which contained
Kahlbaum's C. P. silver nitrate solution, when run in series with
our normal coulometer, the Rayleigh gave a deposit 0.036 percent
heavy. The anode of the coulometer was wrapped in filter paper,
and the anolyte diffused through to the cathode during electrolysis.
7. In our porous-cup coulometers the anolyte was always
lower than the catholyte, and the former did not flow 1 through
except in special experiments, which showed that when the anolyte
is allowed to pass through the porous cup and come into contact
with the cathode a deposit heavier by 0.019 percent was obtained.
This is nearly one-half of the abnormal effect caused by the heavy
anode liquid and filter paper as shown in the Rayleigh form.
8. Experiments were made with thin-walled porous alumina
cups substituted for the porcelain cups. The silver deposits
made using the alundum cups show a heavier deposit in every
case, due to the anolyte passing through to the catholyte during
electrolysis.
I wish to express my great obligation to Professor G. A. Hulett
for having suggested this research and for his kind advice and
counsel in carrying it out.
Laboratory of Physical Chemistry,
Princeton University.
189466
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